In the deep-well drilling art, direct drives are used to deflect the direction of drilling but also for the further drilling of bores which are already inclined. If, in the course of this, the drilling bit jams in the formation for any reason, so firmly that it cannot be released by the torque of the direct drive, then the torque necessary for the release can only be introduced through the rotary table of the derrick and the drill pipe. Since the drill pipe is rotatable in relation to the drilling bit, however, because of the direct drive, a mechanical locking must first be brought about between the drill pipe and the drilling bit before the torque introduced from above can act on the drilling bit.
In a known direct drive of this type (German Patent DE 2,917,331), this is effected by a detent sleeve which is connected to the drive shaft and one which is connected to the bearing housing, which sleeves come into locking engagement through an axial movement exerted on the drilling string. In order that this locking engagement may not occur accidentally through the axial movements occurring in the normal drilling operation, a safety device is present which only releases and frees the axial movement when a certain threshold value is exceeded. The determination of this threshold value is very difficult because if this is set so high that accidental release is reliably avoided, a desired release operation may fail on the other hand, for example because a large proportion of the axial force introduced from above is lost through friction between drill pipe and borehole wall and only a reduced force reaches the safety device.
Furthermore, turbine direct drives are in use in which the locking engagement between the detent sleeves at the stator and rotor side is brought about by a member thrown into the flushing passage from above and entering between the detent sleeves. Here, the detent sleeves are disposed in front of the upstream parts of the turbine. With this arrangement, an adequate stability of the whole of the rotor parts with regard to the torque which can be applied from the rotary table in the locking state is necessary, because these torques are conveyed from the detent sleeves through the rotor parts to the drilling bit.
Although the last mentioned version of a direct drive with locking device renders possible a reliable locking at the desired moment and excludes accidental locking, the use on a direct drive on the Moineau drive principle is not easily possible. On application of a high torque from the rotary table, the rotor would travel radially outwards inside the stator because of the elastomer construction of the one part and would cancel the frictional or positive connection between the detent sleeves brought about by the member thrown in. In addition, the eccentric movements of the rotor inside the stator and the rotation of the rotor, counter to this movement, about its own axis counteract jamming of the thrown-in member between the detent sleeves and hence the occurrence of the locking.
It is an object of the present invention to obviate or mitigate this problem.